Finding A Function From A Power Series

In summary, the problem is asking us to find the sum of the given series and its radius of convergence. The attempt at a solution involves finding the radius of convergence first, and then using the derivative of the series to determine the type of series. By distributing (-1)^(n+1) to (x-1)^(n-1), it can be seen that the resulting series is a geometric series.
  • #1
Bashyboy
1,421
5

Homework Statement


Find the sum of the series and its radius of convergence:

[itex]\sum_{n=1}^{\infty} (-1)^{n+1}\frac{(x-1)^n}{n}[/itex]


Homework Equations





The Attempt at a Solution


I found the radius of convergence, but I wasn't sure how to find the sum of the power series.
 
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  • #2
Bashyboy said:

Homework Statement


Find the sum of the series and its radius of convergence:

[itex]\sum_{n=1}^{\infty} (-1)^{n+1}\frac{(x-1)^n}{n}[/itex]


Homework Equations





The Attempt at a Solution


I found the radius of convergence, but I wasn't sure how to find the sum of the power series.

Hint: If you call that series f(x), what is f'(x)?
 
  • #3
Would it be [itex]f'(x) = \sum_{n=0}^{\infty} (-1)^{n+1}(x-1)^{n-1}[/itex]?
 
  • #4
Bashyboy said:
Would it be [itex]f'(x) = \sum_{n=0}^{\infty} (-1)^{n+1}(x-1)^{n-1}[/itex]?

Yes. And do you recognize what kind of series that is?
 
  • #5
If you distribute the (-1)^(n+1) to the (x-1)^(n-1) would it be a geometric series?
 
  • #6
Bashyboy said:
Would it be [itex]f'(x) = \sum_{n=0}^{\infty} (-1)^{n+1}(x-1)^{n-1}[/itex]?

Bashyboy said:
If you distribute the (-1)^(n+1) to the (x-1)^(n-1) would it be a geometric series?

Do you really have to ask? What do you think? Why? Show us what you would do if it is.
 

1. What is a power series?

A power series is an infinite sum of terms, where each term is a polynomial function of the variable x. The general form of a power series is ∑(an)(xn), where an is a constant coefficient and n is a non-negative integer.

2. How do I find a function from a power series?

To find a function from a power series, you can use the process of power series expansion. This involves substituting the values of x into the power series and simplifying the resulting expression. The resulting function will be the sum of all the simplified terms.

3. What is the difference between a Taylor series and a power series?

A Taylor series is a special type of power series, where the terms are specifically chosen to approximate a given function at a specific point. Power series, on the other hand, have more general terms and do not necessarily have to be centered at a specific point.

4. Can all functions be represented by a power series?

No, not all functions can be represented by a power series. A function must be analytic, meaning it must have a continuous derivative of all orders, for it to be represented by a power series.

5. How can I use a power series to approximate a function?

You can use a power series to approximate a function by truncating the series to a certain number of terms. The more terms you include, the more accurate the approximation will be. This is useful for evaluating complex functions or finding values that are difficult to calculate directly.

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